The overall objective of the proposed research is to elucidate the physiological transport mechanisms involved in the swelling of the blastocyst, an important prerequisite for implantation. Studies will be done on maximally (rabbit) and minimaly (mouse) expanding types of blastocyst. There are five specific aims, all directed towards investigating the molecular mechanisms involved in ion and water transport across the trophectodermal epithelium. Specific aims 1-3 involve the development of a voltage clamp/perfusion system, the study of potassium ion fluxes, and measurement of the distribution and activity of Na/K-ATPase pumps within the trophectoderm. Specific aim four concerns studies designed to determine whether exogenous agents, particularly those likely to arise in the mother, can affect transport across the epithelium. Particular emphasis will be placed upon elucidating the physiological control mechanisms involved in the on/off signaling of various ion transport systems. Fifth intracellular concentrations of physiologically important ions like Na+, K+, Cl+, phosphate, and calcium will be made using electron probe microanalysis. These studies will contribute to our knowledge of the kinetic and biochemical properties of developing ion transport systems in preimplantation mammalian embryos, and increase our comprehension of the physiology of early pregnancy.